Debugging Your Python Code

Visual Studio provides a comprehensive debugging experience for Python, including attaching to running processes, evaluating expressions in the Watch and Immediate windows, inspecting local variables, breakpoints, step in/out/over statements, Set Next Statement, and more.

For a debugging overview, see Getting Started with PTVS, Part 4: Debugging (youtube.com, 3m30s).

In this topic:

Also see the following scenario-specific debugging topics:

Tip

Python in Visual Studio supports debugging without a project. With a stand-alone Python file open, right-click in the editor, select Start with Debugging, and Visual Studio launches the script with the global default environment (see Python Environments and no arguments. But from then on, you have full debugging support.

To control the environment and arguments, you'll need to create a project for the code. You can easily do this with the From Existing Python Code.

Basic debugging

The basic debugging workflow involves settings breakpoints, stepping through code, inspecting values, and handling exceptions as described in the following sections. For complete details on the Visual Studio debugger, see Debugging in Visual Studio.

A debugging session starts with the Debug > Start Debugging command, the Start button on the toolbar, or the F5 key. This will launch your project's startup file (shown in bold in Solution Explorer) with the project's active environment and any command-line arguments or search paths that have been specified in Project Properties (see Project debugging options. If for some reason you don't have a startup file set, however, you'll see a Python output window briefly appear and disappear. In this case, right click the appropriate file and select Set as Startup File.

Note

The debugger always starts with the active Python environment for the project. To change the environment, make a different one active as described on Python Environments.

Breakpoints

Breakpoints stop execution of code at a marked point so you can inspect the program state. They're set by clicking in the left margin of the code editor or by right-clicking a line of code and selecting Breakpoint > Insert Breakpoint. A red dot appears on each line with a breakpoint.

Breakpoints in Visual Studio

Clicking the red dot or right-clicking the line of code and selecting Breakpoint > Delete Breakpoint removes the breakpoint. You can also disable it without removing it using the Breakpoint > Disable Breakpoint command.

Note

Some breakpoints in Python can be surprising for those who are used to other languages. In Python, the entire file is executable code, so Python runs the file when it's loaded to process any top-level class or function definitions. If a breakpoint has been set, you may find the debugger breaking part-way through a class declaration. This is the correct, if sometimes surprising, behavior.

You can customize the conditions under which a breakpoint is triggered, such as breaking only when a variable has reached a certain value. To set conditions, right-click the breakpoint's red dot, select Condition, then create expressions using Python code. For full details on this feature in Visual Studio, see Breakpoint conditions

When setting conditions, you can also set Action and create a message to log to the output window, optionally continuing execution automatically. This creates what is called a tracepoint without having to introduce logging code into your application directly:

Creating a tracepoint with a breakpoint

Stepping through code

Once stopped at a breakpoint, you have various ways to step through code or run blocks of code before breaking again. These commands are available in a number of places, including the top debug toolbar, the Debug menu, on the right-click context menu in the code editor, and through keyboard shortcuts (through not all commands are in all places):

Feature Keystroke Description
Continue F5 Runs code until the next breakpoint is reached.
Step Into F11 Runs the next statement and stops. If the next statement is a call to a function, the debugger stops at the first line of the function being called.
Step Over F10 Runs the next statement, including making a call to a function (running all its code) and applying any return value. Stepping over allows you to easily skip functions that you do not need to debug.
Step Out Shift+F11 Runs code until the end of the current function, then steps to the calling statement. This is useful when you don't need to debug the remainder of the current function.
Run to Cursor Ctrl+F10 Runs code up to the location of the caret in the editor. This allows you to easily skip over a segment of code that you don't need to debug.
Set Next Statement Ctrl+Shift+F10 Changes the current run point in the code to the location of the caret. This allows you to omit a segment of code from being run at all, such as when you know it's faulty or produces and unwanted side-effect.
Show Next Statement Alt+Num * Returns you to the next statement that will be run. This is very helpful if you've been looking around in your code and don't know where the debugger is actually stopped.

Inspecting and modifying values

When stopped in the debugger, you can inspect and modify the values of variables. You can also use the Watch window to monitor individual variables as well as custom expressions. (See Inspect Variables for general details.)

To view a value using DataTips, simply hover the mouse over any variable in the editor. You can click on the value to change it:

DataTips in the debugger

The Autos window (Debug > Windows > Autos) contains variables and expressions that are close to the current statement. You can double-click in the value column or select and press F2 to edit the value:

Autos window in the debugger

The Locals window (Debug > Windows > Locals) displays all variables that are in the current scope, which can again be edited:

Locals window in the debugger

For more on using Autos and Locals, see Inspecting Variables in the Autos and Locals Windows.

The Watch windows (Debug > Windows > Watch > Watch 1-4) allow you to enter arbitrary Python expressions and view the results. Expressions are reevaluated for each step:

Watch window in the debugger

For more on using Watch, see Setting a Watch on Variables using the Watch and QuickWatch Windows.

When inspected a string value(str, unicode, bytes and bytearray are all considered strings for this purpose), you'll see a magnifying glass icon on the right side of the value. Clicking this displays the unquoted string value in a popup dialog, with wrapping and scrolling, which is useful for long strings. In addition, clicking on the drop-down arrow on the icon will allow you to select plain text, HTML, XML, and JSON visualizations:

String visualizers

HTML, XML, and JSON visualizations appear in separate popup windows with syntax highlighting and tree views.

Exceptions

If an error occurs while your program is being debugged, and you don't have an exception handler for it, the debugger breaks at the point of the exception:

Exception popup

At this point you can inspect the program state, including the call stack. However, if you attempt to step through the code, the exception will continue being thrown until it is either handled or your program exits.

The Debug > Windows > Exception Settings menu command brings up a window in which you can expand Python Exceptions:

Exceptions window

The checkbox for each exception controls whether the debugger always breaks when it is raised. You should check this box when you want to break more often for a particular exception.

By default, most exceptions will break when an exception handler cannot found in the source code. To change this behavior, right-click any exception and check or uncheck "Continue When Unhandled in User Code". You should clear this box when you want to break less often for an exception.

To configure an exception that does not appear in this list, click the Add button to add it. The name must match the full name of the exception.

Project debugging options

By default, the debugger starts your program with the standard Python launcher, no command-line arguments, and no other special paths or conditions. These can be changed through the project's debug properties accessed by right-clicking your project in Solution Explorer, selecting Properties, and selecting the Debug tab.

Project debug properties

Launch mode options

The Launch mode options lets you choose between the following options, which enable different scenarios:

Option Description
Standard Python launcher Uses debugging code written in portable Python that is compatible with CPython, IronPython, and variants such as Stackless Python. It provides the best experience for debugging pure Python code. When you attach to a running python.exe process, this launcher is used. This launcher also provides mixed-mode debugging for CPython, allowing you to step seamlessly between C/C++ code and Python code.
Web launcher Starts your default browser on launch and enables debugging of templates. See the Web template debugging section for more information.
Django Web launcher Identical to the Web launcher and shown only for backwards compatibility.
IronPython (.NET) launcher Uses the .NET debugger, which only works with IronPython but allows for stepping between any .NET language project, including C# and VB. This launcher is used if you attach to a running .NET process that is hosting IronPython.

Run options (search paths, startup arguments, and environment variables)

Option Description
Search Paths These match what's shown in the project's Search Paths node in Solution Explorer. You can modify this value here, but it's easier to use Solution Explorer that lets you browse folders and automatically converts paths to relative form.
Script Arguments These are added to the command used to launch your script, appearing after your script's filename. The first item here will be available to your script as sys.argv[1], the second as sys.argv[2], and so on.
Interpreter Arguments These are added to the launcher command line before the name of your script. Common arguments here are -W ... to control warnings, -O to slightly optimize your program, and -u to use unbuffered IO. IronPython users are likely to use this field to pass -X options, such as -X:Frames or -X:MTA.
Interpreter Path Overrides the path associated with the current environment. This may be useful for launching your script with a non-standard interpreter.
Environment Variables In this multi-line text box, add entries of the form NAME=VALUE. This setting is applied last, on top of any existing global environment variables, and after PYTHONPATH is set according to the Search Paths setting, and so it can be used to manually override any of those.

Immediate and interactive windows

There are two interactive windows you can use during a debugging session: the standard Visual Studio Immediate window, and the Python Debug Interactive window.

The Immediate window (Debug > Windows > Immediate) is used for quick evaluation of Python expressions and inspection or assignment of variables within the running program. See the general Immediate Window topic for details.

The Python Debug Interactive window (Debug > Windows > Python Debug Interactive) is richer as it makes the full Interactive REPL experience available while debugging, including writing and running code. It automatically connects to any process started in the debugger using the Standard Python launcher (including processes attached through *Debug > Attach to Process). It's not, however, available when using mixed-mode C/C++ debugging.

Python Debug Interactive window

The Debug Interactive window supports special meta-commands in addition to the standard REPL commands:

Command Arguments Description
$continue, $cont, $c Starts running the program from the current statement.
$down, $d Move the current frame one level down in the stack trace.
$frame Displays the current frame id.
$frame frame id Switches the current frame to the specified frame id.
$load Loads commands from file and executes until complete
$proc Displays the current process id.
$proc process id Switches the current process to the specified process id.
$procs Lists the processes currently being debugged.
$stepin, $step, $s Steps into the next function call, if possible.
$stepout, $return, $r Steps out of the current function.
$stepover, $until, $unt Steps over the next function call.
$thread Displays the current thread id.
$thread thread id Switches the current thread to the specified thread id.
$threads Lists the threads currently being debugged.
$up, $u Move the current frame one level up in the stack trace.
$where, $w, $bt Lists the frames for the current thread.

Note that the standard debugger windows such as Processes, Threads and Call Stack are not synchronized with the Debug Interactive window. This means that changing the active process, thread, or frame in the Debug Interactive window will not affect the other debugger windows, and similarly, changing the active process, thread, or frame in the other debugger windows will not affect the Debug Interactive window.

The Debug Interactive window has its own set of options, which you can access through Tools > Options > Python Tools > Debug Interactive Window. Unlike the regular Python Interactive window, which has a separate instance for each Python environment, there is only one Debug Interactive window and it always uses the Python interpreter for the process being debugged. See Options - Debugging options.

Debug Interactive Window Options